1. Technical Field
The claimed subject matter relates generally to raster images and, more specifically, to a method for converting vector elements to monochrome raster elements.
2. Description of the Related Art
Mathematically-defined vectors have proven to be an efficient method for digitally reproducing human drawings and symbols. Vectors representing points and straight and curved lines presently are used to depict nearly any imaginable shape, including fonts.
Rendering vectors using a personal computing device often has necessitated the conversion of these vector representations to raster representations. With the exception of plotters, a digital device's output displays in raster form, either on a monitor screen or on a printer.
The most common method of converting a vector input to a raster output is to project the vector on to a monochrome raster grid and to turn on those grid pixels touched by the vector. This method causes output pixels to touch at both the sides and at the corners. As a result, diagonal and curved lines often exhibit an unwanted artifact: a stair-step pattern known as aliasing. Especially at low raster resolutions, this jagged representation becomes objectionable when compared to horizontal and vertical lines that do not exhibit the artifact.
A number of “work-a-rounds” have been developed to reduce the visual impact of this aliasing, most notably the technologies of “hinting” and “anti liasing.” Hinting is associated with vector fonts and is an additional instruction set included in font metrics to designate certain raster pixels at low resolutions to best represent the particular font. Anti-aliasing is a method that replaces the monochrome raster with a multi-bit pixel-depth raster that displays grayscale. Shades of gray are used to smooth the abasing stair-step pattern. Both require increases in instruction storage and execution time.
In U.S. Pat. No. 6,002,493, “Method for reproducing an image,” Case demonstrates a way to convert a multi-bit pixel-depth raster to a monochrome raster. His patent describes the use of a temporary raster image in a checkerboard pattern between the input and output stages. The monochrome checkerboard pattern exhibits a smooth gray when viewed from a suitable distance. Case depicts areas of the input image darker than checkerboard gray by turning to black only the white checkerboard pixels in the corresponding area of the output raster image. Areas of the input image lighter than checkerboard gray are depicted by turning to white only the black checkerboard pixels in the corresponding area of the output raster image. Although Case uses a checkerboard, no mention is made of vector to raster conversion.
In U.S. Pat. No. 5,589,851, “Multi-level to bi-level raster shape converter,” Valdez and Martinez demonstrate a vector to monochrome raster conversion that uses a multi-bit pixel-depth raster image between the input and output stages. They recognize the problem of output pixels touching at the sides and recommend an output checkerboard pattern, but do not teach how to ensure that desired output is always achieved. In their FIG. 13C and FIG. 13D, Valdez and Martinez show an optimum output vertical line raster that still allows designated pixels to touch at the sides.